MTN debit problems

I have a contract with MTN. I changed my bank hence I called MTN requesting them to change my banking details. This occured in April. In May my installment was jot debited. When I called MTN the agent said that nothing was processed and she will change the banking details. In June again my debit didnt go through. I called on Wednesday 20 June and the agent says I need to pay R520. I call again on Saturday and they say my account is handed to the legal department. Today morning I call them and they tell me no banking details were changed and they give me a reference number to call back in 3 days. This is so frustrating. I cant be paying additional costs when the fault is with MTN. I need this problem to be sorted out as soon as possible.